When light propagates in a transparent homogeneous medium, it travels along a straight line. Light path can be made to curve if the medium has its optical density (refractive index) changing gradually. In this photograph a diode laser beam (635 nm) propagates through a concentrated solution of sugar in water. The solution was left undisturbed for a while and became more dense at the bottom and gradually less dense toward the surface. This effect is responsible for formation of mirages on a hot day in a desert, except that in the case of a mirage the less dense (hotter) air is near the ground and becomes more dense (cooler) up from the ground.